1. Field of the Invention
The present invention relates generally to control of direct current power to electronic loads, and more particularly to a fully regulated, all electronic switching DC power supply having excellent regulation and low noise.
2. Description of Related Art
The field of electronic power supplies, and in particular electronic switching DC power supplies, has been closely studied for many years and is extremely well developed. As is well known, the primary source of power in such circuits may be from the alternating current power lines (e.g. 120 v, 60 Hz), or from another source of direct current power that is either unregulated or at a voltage level not suitable for the electronic load being supplied. It is a usual requirement of such electronic loads that the voltage supplied be regulated to a fraction of a percent over a wide range of load currents, supply voltages, and operating temperatures. As used herein "unregulated" means the voltage, generally from one of the sources of supply just described, which is to be used as the supply source for the power supply of the present invention.
Generally, regulated power supplies are of two basic types: linear power supplies and switching power supplies (usually called switchers). The linear power supplies are capable of high quality regulating performance, but suffer from relatively low overall efficiency since they regulate the output voltage by deliberately dissipating power in an electronic regulating circuit.
Common methods for providing extra regulation for outputs in linear power supplies which do not have direct feedback to the source are a linear series pass regulator or a switching buck regulator. Examples of these regulators are found in section 9 of "Linear/Switchmode Voltage Regulated Handbook", Motorola Incorporated (1982 Edition) and "TMOS Power FET Design Ideas", Motorola Incorporated (Issue A, 1985 Edition). These types of circuits involve the use of a regulating element, which is usually a transistor. Since the output cirrent flows through the regulating element, a substantial amount of power is thereby dissipated. The regulating elements in the associated heat sink must therefore be of sufficient size and durability to withstand the environment. While these types of regulators are functional, they are undesirable because of the size and cost of the components required and the power wasted in the form of heat.
Switching power supplies usually regulate the output voltage by controlling the duty cycle of the switching transistors that produce an alternating (square wave) voltage at the primary of the main transformer. The transformer, through its normal transformer action, steps the primary voltage up or down to the desired level compatible with the load circuit. Switching power supplies have higher efficiencies than linear power supplies because the transistors that drive the primary winding of the main transformer are either saturated or cut-off. This minimizes the power that is dissipated in these elements since no current flows through the transistors when the voltage is at its maximum value across the transistors, and the maximum current flows through the transistors when the voltage is a minimum (the saturation value) across the transistors. By controlling the duty cycle of the switching transistors, the average value of the transformed voltage can be varied, thus exerting control over the output voltage (and consequently the load voltage) in response to feedback regulating signals from the load.
A circuit for regulation of a DC output voltage in a switching power supply is shown in U.S. Pat. No. 4,375,077, which issued to J. Williams on Feb. 22, 1983. This circuit contains a control transformer having primary, secondary and control windings. The output of the circuit is connected from the primary and secondary windings and is controlled by a switching element connected from the control winding. The voltage output error is provided to the duty cycle modulation circuitry, which varies the duty cycle of the switching element and thereby controls the output voltage level.
Most commercial switching power supplies, as seen in the Williams reference, use a driven type of circuit in which the switching transistor(s) is driven from a completely separate circuit. In addition, these power supplies do not utilize any feedback signals from the main transformer. The duty cycle or pulse width modulation circuitry operates on this separate circuit to provide the required regulating action. An undesired by-product of this pulse width modulation scheme, however, is the production of high frequency noise signals as a result of the narrow, high current pulses induced in the transformer by the switching transistors. Because the main transformer is connected either directly or indirectly to the source of line power, these noise pulses will be carried to the line (or load) if special and expensive filtering techniques are not employed.
Another class of power supplies utilize a current limiting transformer to provide a high voltage that fires a spark gap for igniting liquid or gaseous fuels. The high voltage is required to ionize and break down the spark gap and the current limiting feature limits the current that flows through the gap, which has a relatively low voltage across it once the spark has been formed.
For example, a current limiting power supply for electron discharge lamps which utilizes a transformer like that described above is found in U.S. Pat. No. 4,414,491, which issued to W. Elliott on Nov. 8, 1983. In this arrangement, the power supply includes a high frequency inverter circuit coupled to an electron discharge lamp load through a special purpose transformer. The transformer is wound on a saturable ferromagnetic core structure forming a first magnetic flux path coupling the primary and secondary windings of the transformer and a second shunt magnetic path including an air gap which carries an increasing share of flux as load current increases. The switching of the inverter circuit occurs in response to the partial saturation of the core. Auxiliary windings may be serially connected with the primary winding of the transformer and wound about the shunt magnetic path to enchance the current regulating properties of the supply.
In a similar type of arrangement, U.S. Pat. No. 4,562,382, which issued to W. Elliott on Dec. 31, 1985, illustrates a solid-state inverter which utilizes a multiple core transformer. The multiple core transformer includes a high-permeability saturable core upon which both the primary and secondary windings are wound, and one or more lower permeability non-saturating cores upon which the primary and/or secondary windings are wound to provide additional self-inductance. The inverter switching transistors drive the two halves of the transformer's center-tapped primary winding on alternate half-cycles under the control feedback winding which is wound on the saturable core. A non-saturating core about which the primary winding is wound, and a capacitor connected in parallel with both halves of the primary winding, protects the switching transistors against transients, prevents the saturating core from going into hard saturation, and efficiently transfers energy stored in a leakage inductance of the primary windings from half-cycle to half-cycle. The multicore transformer is assembled by means of bobbin wound primary and secondary windings through which the center leg of ferrite E-core shapes may be inserted to provide the desired saturating and non-saturating flux paths.